This invention relates to drive circuits for d.c. motors and more particularly to such drive circuits which provide for linear motor response to a command signal. The embodiment of the invention illustrated herein is adapted for use in an aircraft autopilot system wherein the motor, in response to electrical commands from an autopilot logic circuit, positions the aircraft control surfaces. Other uses of the invention will also suggest themselves to one skilled in the art.
It is standard practice to power d.c. torquer autopilot servomotors which position the aircraft control surfaces in the aircraft flight control systems from the aircraft primary d.c. voltage supply. The need for reversible servomotors in such systems suggests that a bridge motor drive circuit be used. In these systems the d.c. servomotor is connected across interior points of the bridge and the aircraft primary d.c. voltage supply is connected across the bridge. The bridge, under present practice, is comprised of active power elements such as power transistors. It is highly desirable that the bridge and its associated control elements be designed to prevent both sides of the bridge from turning on to destroy the power transistors.
The present invention provides a drive circuit for d.c. servomotors which completely eliminates the aforementioned self-destructive mode of operation and further provides for direct control of servomotor current and thus direct control of motor torque. This is accomplished by providing a differential voltage across the motor, by means of a full transistor bridge. Each side of the bridge is controlled by an associated operational amplifier, both of which are referred to the same reference voltage, which is one-half the voltage across the bridge, by means of a voltage divider which is common to both operational amplifiers. A first of the operational amplifiers drives one side of the bridge in response to an input motor control signal, while the other operational amplifier drives the other side of the bridge in response to the voltage developed across the motor and, in effect, provides a feedback command. Since both operational amplifiers are referenced to the same voltage there will be no quiescent voltage difference across the motor.
A current drive loop includes a comparator which provides an input current command signal which is a direct torque command to the motor and responds to the difference between the currents in the various sides of the bridge and the input motor command signal.